1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a device incorporating a micro-porous membrane for venting gases from a seal assembly of a reactor coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Pat. Nos. to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222) and Boes (4,275,891) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The lower primary sealing assembly (or No. 1 seal), the main seal of the pump, is a controlled-leakage film-riding face seal. Its primary components are a runner which rotates with the pump shaft and a non-rotating seal ring which is attached to the housing of the lower seal assembly. The No. 1 seal causes a pressure drop of the coolant from about 2250 psi to 30 psi across its face and allows a flow rate of about 2-3 gpm therethrough. The low-pressure coolant leaking through the No. 1 seal flows up the shaft annulus to the region of the middle secondary sealing assembly.
The middle secondary sealing assembly (or No. 2 seal) is a rubbing face-type seal. Its primary components are a rotating runner and a non-rotating ring. Most of the coolant from the No. 1 seal is diverted by the No. 2 seal out through the No. 1 seal leakoff. However, a portion of the coolant passes through the No. 2 seal, leaking at a flow rate of about 2 gph at a pressure drop of from 30 psi to 3-7 psi. The lower pressure coolant leaking through the No. 2 seal flows further up the shaft annulus to the region of the upper tertiary sealing assembly.
The upper tertiary sealing assembly (or No. 3 seal) is also a rubbing face-type seal, its primary components being a rotating runner and a non-rotating ring. Most of the coolant flow leaking from the No. 2 seal is diverted by the No. 3 seal out through the No. 2 seal leakoff. However, a small portion of the coolant passes through the No. 3 seal.
In one form, the No. 3 seal has a double face or dam with two concentric sealing faces. Coolant is injected into the annulus between the two faces or dams such that one portion of the injected coolant flow goes back outward past the outer or upstream one of the seal dams into the housing cavity between the No. 2 and No. 3 seals and then out the No. 2 seal leakoff, while another portion of the injected coolant flow goes in the opposite direction inward past the inner or downstream one of the seal dams and ultimately through the No. 3 seal leakoff to the containment atmosphere.
A centrifugal pump in the form of flow passages incorporated in the rotating runner of the No. 3 seal act to feed coolant to the annulus between the seal dams. Each passage has an inlet located on a smaller radius of rotation relative to the shaft axis than an outlet of the passage. Rotation of the runner causes a pressure differential in the reactor coolant flowing within and along the passages due to centrifugal effects.
The reactor coolant is comprised primarily of water. The coolant water contains dissolved boron, which is used as a moderator in the nuclear reaction, and corrosion inhibitors. Dissolved gases may also be present in the coolant water either added intentionally to control water chemistry or as residual amounts from fluid processing. As the coolant water passes through the above-described Nos. 1, 2 and 3 seals and undergoes the associated drops in pressure and absorbs heat from friction, dissolved gases tend to come out of solution and are released in the form of small bubbles. These bubbles can collect in high spots in the seal housing which may not be adequately vented due to the elevation and radial location of the leakoff port relative to the free surface generated by the fluid rotating within the seal housing. The venting of gases may also be aggravated by trap regions which may have been formed in the layout and design of connecting system piping.
Excessive accumulation of gases can detrimentally affect seal performance. If the free surface of coolant water does not cover the inlet of a seal, its function may be hampered by inadequate cooling and lubrication or decreased leakage. In the case of the No. 3 seal incorporating passages in its rotating runner which act as a centrifugal pump, the collection of gases can cause additional problems If gases accumulate to a point which does not permit access of coolant water to the pumping passage inlets, the gases will supplant the coolant water in the passages and drastically reduce the head developed through centrifugal action.
Consequently, a need exists for an approach to venting the seal housing of the reactor coolant pump so as to avoid the potential deleterious effects which can arise from accumulation of gases released from the coolant water due to the drop in pressure thereof in the pump.